A hydraulic hose is mainly used for a hydraulically operated machine of which examples include: a construction machine such as an excavator, a bulldozer; an industrial machine such as a hydraulic jack, a hydraulic puncher, a hydraulic press, a hydraulic bender; and the like. A hydraulic hoses in such a machine as described above plays an important role of transmitting driving force by way of pressure of hydraulic oil charged in the hose.
It is therefore essential that a hydraulic hose exhibits satisfactorily high oil resistance and relatively small volume expansion under pressure, as well as satisfactorily high pressure resistance and high ability of accurately and swiftly transmitting driving force (pressure). Further, a hydraulic horse must have satisfactorily high heat resistance and high durability, as well, because it is often used under a harsh environment where vibrations, bending, and the like are constantly applied.
A hydraulic hose 1, due to such characteristics required thereof as described above, generally has a laminated structure as shown in FIG. 1, where an inner tube rubber layer 2 made of rubber (the layer may occasionally be referred to as an “inner tube rubber” hereinafter) in which hydraulic oil is internally charged, a reinforcing material layer 5 for bearing pressure from the hydraulic oil, a woven material underlayer 3 (the layer may occasionally be referred to as a “woven base material” hereinafter) for protecting an unvulcanized inner tube rubber layer in the manufacturing process, and an external cover rubber layer 6 (the layer may occasionally be referred to as an “external cover rubber” hereinafter) for preventing the reinforcing material layer 5 and the inner tube rubber layer 2 from being damaged by external force, are sequentially laminated in this order. An intermediate rubber layer 4 is often provided under the reinforcing material layer 5 and it is common to provide a plurality of pairs of the reinforcing material layer 5 and the intermediate rubber layer 4 according to necessity.
The inner tube rubber layer generally uses acrylonitrile butadiene rubber (NBR) excellent in oil resistance and heat resistance and a thiuram-based compound as a vulcanization accelerator in order to improve adhesion properties thereof. Further, the reinforcing material layer generally uses a brass-plated wire as a reinforcing material (refer to PTL1, for example).
Yet further, although a case where hydraulic oil is charged in a hydraulic hose for use was described above, there is also a case, depending on an application of a hydraulic hose, where water-based hydraulic fluid is charged in the hydraulic hose. In the case where water-based hydraulic fluid is charged in a hydraulic hose, different from the case where hydraulic oil is charged therein, there exists a possibility that moisture contained in the water-based hydraulic fluid permeates into the hose, thereby corroding a reinforcing material of the reinforcing material layer. It is necessary in this regard to suppress such corrosion of a reinforcing material in the reinforcing material layer.